cocoa: Handle VulkanSurface for backingstores

Should be no different from MetalSurface and OpenGLSurface.

Change-Id: I529f2904a43e44376e9f2da2489fac57670d954d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit e310bc1646eab51b70eb68e7d4ed344dd263167f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/cocoa/qcocoaintegration.mm

@@ -309,6 +309,7 @@ QPlatformBackingStore *QCocoaIntegration::createPlatformBackingStore(QWindow *wi
         return new QCALayerBackingStore(window);
     case QSurface::MetalSurface:
     case QSurface::OpenGLSurface:
+    case QSurface::VulkanSurface:
         return new QRhiBackingStore(window);
     default:
         return nullptr;